In a Wireless Local Area Network (WLAN) an Access Point (AP) can communicate with a plurality of member stations over a wireless communications channel. Each member station may be, for example, a cellular telephone, a portable computer, or any other wireless receiver and/or transmitter unit.
Generally, each of a plurality of member stations of a Basic Service Set (BSS) in a Wireless Local Area Network (WLAN) communicates with other stations and with an Access Point (AP) of the same BSS using either a scheduled access approach or a contention based access approach. In a scheduled access approach each member station is polled in a round robin fashion by the AP to allow that member station to transmit while other member stations cannot. In a contention based approach each member station checks a communications channel to see if it is busy. If the communications channel is not busy the member station attempts to transmit. If the communications channel is busy, the member station defers its transmission for a period of time, which typically includes an Arbitration Inter-Frame Space (AIFS) followed by a Contention Window (CW). If the communications channel is not busy for the entire deferral period, the member station then can begin its transmission.
Communications between an Access Point (AP) and a member station may include control and management signals, messages, or traffic flow, such as beacon signals, as well as data signals, messages, or traffic flow, such as Voice over Internet Protocol (VoIP) calls. Control and management signals are typically independent of, and may interfere with, data signals. Generally scheduling methods have only considered service cycles of admitted QoS data traffic flows because in systems such as cellular systems for which most scheduling methods have been developed, control and management signals are typically transmitted on a separate control channel instead of sharing the data channel with QoS data traffic. However, in WLAN systems, control and management signal transmissions share the same channel as data transmissions. Because control and management messages and signals are often not periodic, or even when they are periodic their service cycles are very different from QoS data service cycles, how to schedule both control and management traffic and QoS data traffic efficiently is not trivial in WLAN systems.
An AP controls and coordinates data transmissions of its member stations' QoS streams by using a service schedule, which schedules data transmissions for admitted QoS streams in a round robin and periodic fashion. Each scheduled transmission is referred to as a service task. The service cycle duration is referred to as schedule duration.
One method for scheduling QoS stream data transmission in WLAN systems involves reducing a beacon interval to the same as the minimum of all maximum service intervals of all admitted QoS data streams. The maximum service interval of a QoS data stream is normally determined as the maximum transition delay that the QoS data stream can tolerate. For instance, a VoIP (Voice Internet Protocol) data stream may have a packet generation interval of ten ms (milliseconds) and a maximum service interval of fifty ms. This means that even if a packet is delayed for fifty ms in transit, when it arrives at a final receiver end, the data within the packet is still usable. On the other hand, a reference scheduler proposed by the IEEE 802.11e-2005 document (Section K3.3.1) suggests building a simple schedule with Scheduled Service Interval (SSI) being a number that is both a submultiple of the beacon interval and less than the minimum of all maximum service intervals of all admitted streams.
Both of the above approaches are similar in the way that the schedule duration is based on the minimum of all maximum service intervals of all admitted streams. A common problem for both approaches is that the beacon transmissions are not handled efficiently. The first approach produces unnecessary beacon transmissions, which wastes valuable air time that can be used by data transmissions. Although the second approach does not produce unnecessary beacon transmissions, it still needs to leave a time slot within each SSI so that beacon transmissions can be accommodated, despite the fact that they only occur once every several SSI's. Because such a time slot is not available in every SSI, even when it is unoccupied by a beacon transmission it can not be used by QoS stream data. At best it can be used by best-effort non QoS data. In other words, the second approach is still not efficient in terms of accommodating QoS stream data such as Voice over Internet Protocol (VoIP) data streams.
Generally, the order in which service tasks are executed does not change from cycle to cycle. A scheduler, such as an Access Point (AP) may rearrange service tasks when new service tasks are accepted and added to the schedule, or when existing service tasks or traffic flows are terminated and their corresponding service tasks are withdrawn from the schedule. These events are typically infrequent.
However, U.S. published patent application no. 2007/0097867, to Kneckt et. al. published on May 3, 2007, discloses techniques for providing a changed data transmission schedule. A first wireless station in a wireless network receives a first data transmission schedule. The first wireless station attempts to transmit data according to the first data transmission schedule. Subsequently, the first wireless station transmits a request for a schedule change, and receives a second data transmission schedule. In Knecket, an Access Point (AP) may provide changed data transmission schedules to one or more stations in order to relocate the data transmission or contention start times for one or more traffic streams in order to relieve congestion or prevent data collisions.